Single crystal silicon rods



June 3, 1969 5. T ET AL 3,447,902

SINGLE CRYSTAL SILICON RODS Filed April 4. 1966 INVENTORS Theodore S. Benedict Howard W. Norman United States Patent Office 3,447,902 Patented June 3, 1969 3,447,902 SINGLE CRYSTAL SILICON RODS Theodore S. Benedict and Howard W. Norman, Scottsdale, Ariz., assignors to Motorola, Inc., Franklin Park, 11]., a corporation of Illinois Filed Apr. 4, 1966, Ser. No. 539,870 Int. Cl. C01b 33/02 U.S. Cl. 23-2235 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the formation of single crystal semiconductor bodies and more particularly, to silicon rods of a predetermined crystallographic orientation having a polyhedral form made directly by vapor deposition.

The use of single crystal semiconductor material in the form of wafers for making rectifiers, transistors, integrated circuits, and other such elements, devices and computer circuits is well known in the art. Generally such single crystal semiconductor wafer material is prepared by slicing a single crystal rod of the same material transverse to its longitudinal direction. The single crystal rod in turn usually is prepared by forming a melt of polycrystalline material and ulling the rod from the melt. The polycrystalline material in the melt is prepared by vapor deposition. Thus, three separate steps are needed.

It would be preferable to form the single crystal rod directly from the vapor phase by deposition onto a single crystal substrate rod. This method would eliminate the additional step of growth from the melt, which is difl'icult to control with respect to crystallographic orientation and crystal twinning. However, in the present state of the art, it is extremely difiicult to prepare good single crystal semiconductor rods by vapor-deposition. In particular such growth has a pronounced tendency to be polycrystalline. Even when a given run produces a single crystal deposit, it is usually not of a high degree of crystalline perfection. Wafer slices cut from such rods thereby do not have the mirror-like surfaces characteristic of a more nearly perfect single crystal. Thus, it has been found necessary to treat such surfaces by elaborate etching and polishing procedures in order to provide the degree of smoothness necessary for fabricating semiconductor devices on the surface of the crystal.

Various methods have been proposed in the literature to overcome these deficiencies. For example, US. Patents Nos. 3,222,217, 3,021,198 and 3,212,922 describe apparatus and methods for growing single crystal semiconductor material, particularly silicon, from the vapor phase. In addition, US. Patents Nos. 3,172,791 and 3,226,269 describe the preparation of monocrystalline elongate polyhedral semiconductor material by deposition of mono crystalline semiconductor material onto a cylindrical monocrystalline semiconductor core rod. Growth is continued until the cylindrical core is transformed into a polyhedral form. Then still more material of a different conductivity is deposited to form a plurality of transition or junction regions within a unitary structure. Thus, devices and circuits were built into a rod directly. In this method, the core seed is oriented so that the plane perpendicularly transverse to the longitudinal axis of the seed rod has Miller indices such that two of the integers are the integer one, and the sum of all three integers, disregarding any negative signs, does not exceed four, i.e., a (110) plane, a (211) plane, etc. Under these conditions, single crystal, regular polyhedral bodies are grown; in particular, a regular hexagon, is the ultimate product of the process. However, the technique is necessarily restrictive in crystallographic orientation of the product, and the single crystal bodies do not possess the high degree of crystalline perfection suitable for direct use without further purification in semiconductor industry applications.

Accordingly, it is an object of the present invention to provide an improved method of growing single crystal semiconductor material directly from the vapor phase.

Another object of the instant invention is to provide a method of making single crystal silicon rods of a high degree of crystalline perfection by growth from the vapor phase onto a single crystal silicon substrate rod.

Still another object is to provide a single crystal, elongate, polyhedral semiconductor body having a high degree of crystalline perfection.

A more specific object of the invention is the provision of an elongate single crystal silicon semiconductor body having a polyhedral form from which slices cut parallel to the longitudinal axis of the elongate body have a mirror-like surface finish.

A feature of the present invention is the employment of a cylindrical substrate rod of single crystal semiconductor material having its longitudinal axis crystallographically oriented in the direction for deposition of single crystal semiconductor material thereon in an improved manner.

Another feature of the invention is the provision of a single crystal semiconductor rod of a high degree of crystalline perfection made directly from the vapor phase, whose cross section is in the form of an elongate polyhedral body having 4n surfaces, where n is at least 2, on a section perpendicularly transverse to the longitudinal axis of the rod.

These and other objects and features of the invention will be made apparent from the following more particular description of the invention in which reference will be made to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a suitable apparatus for use herein;

FIG. 2 is an elongate, cylindrical, single crystal semiconductor substrate rod crystallographically oriented according to the present invention;

FIG. 3 shows a polyhedral body of the invention at an intermediate stage in the process of growth from the vapor phase; and

FIG. 4 illustrates the polyhedral body at the completion of growth according to the invention.

The present invention is embodied in a process of making semiconductor material by deposition from a vapor source of decomposable semiconductor material upon the surface of a heated semiconductor substrate rod, characterized by the improvement of providing as the semiconductor rod a substantially cylindrical rod of essentially single crystal material having its longitudinal axis crystallographically oriented in the [100] direction so that the plane perpendicular to the longitudinal axis is the (100) plane. The semiconductor material is de posited in single crystal form upon such a substrate until the substrate rod assumes a polyhedral form having 4n surfaces, where n is at least 2, on a section perpendicularly traverse to the longitudinal axis of the rod. The 4n surfaces are planar and essentially parallel to the longitudinal axis of the rod.

In accordance with the invention there is provided a single crystal elongate semiconductor body having the above-described crystallographic orientation, a polyhedral form and an elongate or rod-shaped structure. Such bodies exhibit a high degree of crystalline perfection and are particularly useful in device application without further purification or surface treatment.

In a preferred form of the invention the polyhedral body is an octagon, i.e., n=2, and the semiconductor material is silicon. Four of the angles formed by the intersecting surfaces of the octagon are about 171 and the other four, in alternative arrangement with the large angles, are about 99. The planar surfaces of the octagon are disposed around the longitudinal axis of the rod at a substantially equal distance therefrom measured along lines from the longitudinal axis perpendicular to the respective planar surfaces.

An intermediate product formed during vapor deposition is a dodecagon, i.e., n=3. Four of the angles in this polygon are about 171, and the remaining eight, in alternate arrangement, are about 139. Eight of the planar surfaces are substantially equal, and the remaining four are equal to each other but smaller in size than the other surfaces.

Referring now to FIG. 1, there is shown in schematic illustration a suitable apparatus for vapor growth of semiconductor material according to the invention. This form of apparatus is shown for illustrative purposes only, and it will be understood that any other apparatus and method known in the art for growing semiconductor material from the vapor phase onto a rod-shaped substrate may be used as well. The apparatus includes a reaction chamber 10, such as a quartz vessel, in which is disposed an elongate single crystal, semiconductor substrate body 11, uch as a silicon rod. Future references to semiconductor material herein will be made with respect to silicon, although it will be appreciated that other semiconductor materials also may be used. The silicon rod 11 is mounted in the reaction vessel '10 in conductive chucks 12 and 120 which are positioned at opposite ends of the vessel. An electrical voltage 13 is connected across rod 11 to apply a current through the rod to heat the rod to an elevated temperature suitable for decomposing a gaseous source of silicon thereon, usually about 1100 to 1200 C. A gas inlet port 14 is provided for admitting a decomposable silicon compound into the reaction vessel. Usually the silicon compound is a silicon halide, such as silicochloroform or silicon tetrachloride. Preferably it is a mixture of silicochloroform and silicon tetrachloride, in hydrogen. The spent reactant gases are removed from the reaction chamber through exhaust port 14a. The reactant mixture decomposes on the heated rod liberating silicon which deposits on and increases the diameter of the rod.

Since silicon semiconductor material has a negative resistance temperature coefficient, that is, a high resistance to passage of electrical energy (when cold) it is necessary to initially provide a means other than the normal voltage, to heat the silicon from room temperature to an elevated temperature. Several techniques may be employed for this purpose. For example, it is known in the art to preheat the silicon body by applying an excessively large voltage in order to get the body to heat up and conduct. Alternatively, one can initially heat the silicon with a source of radiant energy applied externally through the walls of the reaction chamber. While either of these methods may be used, it is preferable to employ a technique described in the copending application of Theodore S. Benedict and Albert E. Ozias, Ir., Method for Making Silicon, Ser. No. 540,018, filed Apr. 4, 1966. In this method, a conductive shell is first formed on the silicon rod by diffusing impurities, for example, phosphorus, into the outer surface of the rod. The rod is thereby rendered conductive at room temperature and can be heated directly from room temperature to the deposition temperature. Before the reactant mixture is introduced into the reaction chamber, however, the conductive shell may be removed by gas etching, suitably with about 4 2-5% HCl in H at 1100 C. for 15 minutes. Then deposition of silicon can proceed in the usual manner. During deposition, various doping impurities can be added to the reactant mixture to provide single crystal material of a particular conductivity and resistivity profile.

The reactor apparatus and reactant mixture described above is to be considered merely illustrative of a single stage reactor for depositing the desired silicon material from the vapor phase. A preferred system for making silicon in a cyclic process has been described in the copending application of Theodore S. Benedict and Rudolph Steckl, System for Making Silicon, Ser. No. 539,955, filed Apr. 4, 1966. In this method of mixture comprising about 8 mole percent silicochloroform and 4 mole percent silicon tetrachloride in hydrogen is passed through a reactor, and the spent gases, with the HCl by-product removed, are recirculated through the reactor. Additional silicochloroform is added to replenish the silicochloroform decomposed in the reactor. The silicon tetrachloride reactant is generated internally in the system as a byproduct of the decomposition of the silicochloroform.

The substrate '11, as shown in FIG. 2, is provided in the form of a cylindrical rod of single crystal silicon. The rod is obtained by pulling the crystal from a molten mass of silicon, or by drawing a thicker rod down to a smaller diameter. Usually the rod is about 36 inches long and about 77 mm. in diameter. The single crystal rod is pulled or drawn so that it is crystallographically oriented in a predetermined manner according to the practice of the invention. The desired orientation is accomplished, for example, by providing a seed crystal during pulling from the melt such that the longitudinal axis of the rod is crystallographically oriented to the direction of the axis. The directions transverse to the [100] direction of the crystal then are [001] and [010] directions, as shown in FIGS. 3 and 4. Orientation of the substrate crystal in this manner assures the desired polygonal crystal growth, as will be described in detail hereinafter.

Referring now to FIG. 3, there is shown the polygonal body formed by depositing silicon on the cylindrical core rod oriented in the [100] direction. The body is still elongate with its central core being the cylindrical rod, but it possesses also a polyhedral cross section perpendicularly transverse to the longitudinal axis of the rod. The polygonal body is characterized by having 4n surfaces where n is at least 2. Each of the surfaces of the body are planar and are essentially parallel to the longitudinal axis of the rod. Since the cylindrical rod is crystallographically oriented with its longitudinal axis in the [100] direction, the longitudinal axis of the polygonal body is also crystallographically oriented in the [100] direction.

The polygonally-shaped body in FIG. 3 is a twelvesided polygon or dodecagon, i.e., n=3. The dodecagonshaped product is a non-regular polygon having four angles each of which is about 171, and eight angles of about 139 each. The eight planar surfaces 15 of the dodecagon are substantially equal in size and larger than the remaining four plane surfaces 16 which also are of substantially equal size.

Upon continued deposition of silicon, an octagon-shaped polygonal body is formed as shown in FIG. 4. There are 4n surfaces in this body where n=2. Four of the angles of the octagon are about 171, the remainder are about 99, the larger angles alternative with the smaller angles around the octagon. The planar surfaces 17 of the octagon are disposed around the longitudinal axis of the rod at a substantially equal distance therefrom measured along lines from the longitudinal axis perpendicular to the respective planar surfaces of the polygon. Further desposition of silicon merely results in an enlargement of the outer surfaces of the body.

The polygonal body of the invention may be cut in a direction perpendicularly transverse to the longitudinal axis of the crystal to provide wafers having [100] plane faces exposed. Such wafers exhibit a high degree of crystalline perfection and are eminently suitable for use in making semiconductor devices without further purification.

What has been described herein is a method for making elongate polygonally-shaped semiconductor bodies. Essentially the technique involves the use of a cylindrical single crystal semiconductor rod With its longitudinal axis crystallographically oriented in the [100] direction as the substrate crystal for deposition of silicon thereon from the vapor phase. The invention has been illustrated with particular reference to silicon as the semiconductor material. The product of such a process is an elongate silicon body having a finite length along its longitudinal axis, a central, generally rod-shaped cylindrical core of single crystal silicon and a polyhedral form on a section perpendicularly transverse to the longitudinal axis. The polygonal body has 411 planar surfaces, where n is at least 2 on a section perpendicularly transverse to the longitudinal axis of the rod. In a preferred form of the invention the final product of continued deposition is an octagon. An intermediate product of the deposition is a dodecagon. Wafers cut from these bodies have [100] plane surfaces exposed which are of a high degree of crystalline perfection.

While the invention has been described with particular reference to certain embodiments thereof, it will be understood that changes and modifications may be made which are within the skill of the art. It is intended to be limited only by the following claims.

We claim:

1. A single crystal elongate polyhedral silicon body having:

(a) a finite length along the longitudinal axis of a central, generally rod-shaped, cylindrical silicon core, and a polygonal cross section perpendicularly transverse to the longitudinal axis, including angles of about 139 and 171,

(b) the longitudinal axis of said core being crystallographically oriented in the [100] direction so that the plane perpendicular thereto is the (100) plane,

(c) the polyhedral body having 4n surfaces, where n is at least 2, each of said surfaces being planar and essentially parallel to said longitudinal axis at a substantially equal distance therefrom measured along lines perpendicular to the respective planar surfaces.

2. A single crystal alongate polyhedral silicon body having:

(a) a finite length along the longitudinal axis of a central, generally rod-shaped, cylindrical silicon core, and a polygonal cross section perpendicularly transverse to the longitudinal axis, including angles of about 99 and 171",

(b) the longitudinal axis of said core being crystallographically oriented in the [100] direction so that the plane perpendicular thereto is the (100) plane,

(c) the polyhedral body having 4n; surfaces where n is at least 2, each of said surfaces being planar and essentially parallel to said longitudinal axis at a substantially equal distance therefrom measured along lines perpendicular to the respective planar surfaces.

References Cited UNITED STATES PATENTS 2,961,305 11/1960 Dash 23-2235 XR 3,128,154 4/1964 Bean et al. 23-2235 3,168,422 2/1965 Allegretti et al. 23-2235 XR 3,172,791 3/ 1965 Allegretti et al. 23-2235 XR 3,200,001 8/1965 Merkel et al. 23-2235 XR 3,366,462 1/1968 Kersting et al. 23-2335 XR OTHER REFERENCES Lawson and Nielsen book Preparation of Single Crystals, 1958 ed., pp. 241-244 inclusive. Butterworths Scientific Publications, London.

EDWARD STERN, Primary Examiner.

US. Cl. X.R. 

